I took a break to grab a growler from my local brewery (Singlecut) and when in hopped back on my bike after riding for about 5 minutes I heard a large POP. I tore open my backpack and found the growler open and beer everywhere. Was it the bike riding? Would this have happened with a glass growler? I've taken a picture of the offender and the backpack.

I have had screw tops under pressure that "pop" when I have about a quarter turn left. I am assuming as the threads taper towards the end the pressure overcomes the friction of the threads. I own a growler just like that an I think it would be highly unlikely for the top to go from snuggly screwed on to popping completely off... But I guess there is no way to tell without a controlled experiment.

I know with that growler you really have to screw the lid on extra extra tight. I also did a comparison from that ss growler and a glass growler and it holds about 6oz less than glass. It sure keeps things super cold super long.

I thought I had the lid on really tight but next time I'll have to really give it a twist. All I can think is maybe there was a bit too much headroom and the beer had room to shake around. I'll also probably be a bit more careful next time and maybe use my panniers.

I've used this growler In a backpack a lot, just never while riding my bike. Could the beer being highly carbonated also play a roll?

I carry a pretty similar one in my panniers all the time, and don't have any problems with the top, as long as it's not cross-threaded or anything. I guess my beers aren't very highly carbonated, though (I tend to like them kinda English-y, under 2.0 volumes).